This disclosure relates generally to magnetic resonance imaging (MRI) techniques, and in particular to techniques for separation of water and fat signals during spin-lock MRI. Such techniques can be used, for example, in T1rho imaging and quantification as well as other imaging biomarkers.
Magnetic resonance imaging (MRI) is a noninvasive diagnostic technique that can allow assessments of the composition and state of various tissues. In an MRI procedure, a patient is placed in a strong longitudinal magnetic field (B0) that aligns nuclear spins of atoms in the patient's body, producing a net magnetization vector. Radiofrequency (RF) pulses with magnetic field components (B1) transverse to the longitudinal field and frequencies tuned to the Larmor frequency of an isotope of interest (often 1H) are applied. These pulses can flip spins into a higher energy state, resulting in a transverse component to the magnetization vector. As these spins return to the ground state, responsive magnetic resonance signals from the patient's body can be detected. Based on these signals, characteristics of the magnetization can be measured.
Spin-lock techniques in MRI generally involve applying a long RF pulse (referred to as a “spin-lock” pulse) to lock the magnetization around an effective magnetic field. Such techniques can be used to quantify various imaging biomarkers that may reveal helpful information as to the macromolecular content of tissue. For instance, the spin-lattice relaxation time in the rotating frame (T1rho, or T1ρ) characterizes the decay (or relaxation) rate of spins during the spin-lock process. Because a number of diseases begin to alter the macromolecular content of tissue at a very early stage, spin-lock MRI offers the potential for early detection of disease. In addition, spin-lock MRI can potentially be used to monitor the effectiveness of treatment at the macromolecular level.
Conventional spin-lock MRI techniques are highly susceptible to the presence of fat, in part because protons in fat molecules have a chemical shift that can cause failure of spin-lock. For tissues that include infiltrative fatty tissue, this can result in artifacts and quantification errors. To reduce such artifacts and errors, spectrally selective RF pulses have been applied (e.g., prior to the spin-lock pulse) to suppress the fat signal. This approach is limited, however, in part because the approach is susceptible to B0 field inhomogeneity (which is common in modern MRI systems) and in part because fat has multiple chemical shift components, each with a different chemical shift, and a spectrally selective RF pulse cannot suppress all of these components.
Accordingly, improved techniques for fat suppression during spin-lock MRI would be desirable.